Resistive based NOx sensing method and apparatus

ABSTRACT

Nitrous oxide (NOx) sensors and related methods and systems for use with combustion processes. The NOx sensor uses metal oxide semiconductors. The NOx sensor may have two sensing circuits that share a common electrode. The sensing circuits are differentiated by having different porous catalytic filter coatings protecting the metal oxide semiconductors: one sensing circuit has a porous catalytic filter coating that contains a Noxcat material (rhodium, ruthenium, cobalt, palladium, or nickel), while the porous catalytic filter coating of the other sensing circuit is substantially free of Noxcat material. The two sensing circuits are simultaneously exposed to the exhaust gases at a common macro location. The NOx level may be determined based on a difference in resistance between the two sensing circuits and a temperature of the NOx sensor.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/312,537, filed 24 Mar. 2016, the entire disclosure of which isincorporated herein by reference.

BACKGROUND

This application is related to nitrous oxides sensors, methods of usingnitrous oxides sensors, and related systems for use with combustionprocesses, for example in internal combustion engines and burnersystems.

Nitrous oxides often form in internal combustion engines and burnersystems under high temperature conditions. These are primarily NO, NO₂and N₂O, and are collectively referred to as NOx. NOx formation is aparticular issue in diesel engines, which run leaner and at highercompression ratios than typical spark ignition engines. These nitrousoxides, when released to the atmosphere, may combine with moisture toform nitric acid, leading to adverse health and environmentalconsequences. To combat this issue, modern engines and burner systemsmay be equipped with selective catalytic reduction systems which trapthe NOx then react them with ammonia (NH₃) to form N₂ and H₂O. Thisammonia is typically introduced into the system in the form of urea(CO(NH₂)₂), which converts to ammonia when heated in the exhaust stream.Nitrous oxide sensors are used to control the performance of suchsystems. These nitrous oxide sensors are placed after the NOx trap(typically a Selective Catalytic Reduction (SCR) catalyst), and once NOxis detected, the system is triggered to purge the trap using the urea.These nitrous oxide sensors should be durable and be capable ofdetecting NOx in the 1 to 2000 ppm range, and not experience crosssensitivities to other gasses (H₂, CO, sulfur and sulfur compounds,phosphorus and phosphorous compounds, etc. . . . ). Many NOx sensors onthe market today are electrochemical sensors based on ion conductortechnology; such NOx sensors have not proven satisfactory for allsituations.

As such, there remains a need for alternative type of NOx sensors, andrelated systems and methods.

SUMMARY

This application is related to NOx sensors, methods of using NOxsensors, and related systems for use with combustion processes, forexample in internal combustion engines. In particular, the applicationrelates to NOx sensors that that have metal oxide semiconductor-basedsensor portions, and related systems/methods. The NOx sensor may be usedto sense NOx levels when exposed to exhaust gases of a combustionprocess.

In general, the NOx sensor may have two sensing circuits that share acommon electrode. The sensing circuits contain the same metal oxidesemiconductor material, but are differentiated by having differentporous catalytic filter coatings thereon: one sensing circuit has aporous catalytic filter coating that contains a nitrous oxide catalyzingor “Noxcat” material (e.g., rhodium, ruthenium, cobalt, palladium, ornickel), while the porous catalytic filter coating of the other sensingcircuit is substantially free of the Noxcat material. The two sensingcircuits are simultaneously exposed to the exhaust gases at a commonmacro location. The NOx level can be determined based on a difference inresistance between the two sensing circuits and the temperature of theNOx sensor. The difference in resistance between the two sensingcircuits is a result of the different amount of oxygen that therespective metal oxide semiconductor materials are exposed to due to thecatalyzing action of the Noxcat material containing catalytic filtercoating. The NOx values may be used to control an operation of theexhaust control system, such as triggering a purging of the NOx trap(e.g., providing urea/ammonia thereto) when the NOx level exceeds athreshold.

In one aspect, a nitrous oxide sensor has a substrate; a first oxygensensing circuit affixed on a first side of the substrate; and a secondoxygen sensing circuit affixed on the first side of the substrate. Thefirst oxygen sensing circuit comprises a first sensing electrode; acommon electrode; and a first metal oxide semiconductor portionelectrically bridging a first physical gap between the first sensingelectrode and the common electrode. The second oxygen sensing circuitcomprises a second sensing electrode; the common electrode; and a secondmetal oxide semiconductor portion electrically bridging a secondphysical gap between the second sensing electrode and the commonelectrode. A first catalytic filter layer is disposed over the firstmetal oxide semiconductor portion, with the first catalytic filter layercomprising a Noxcat material. The Noxcat material is one of rhodium,ruthenium, cobalt, palladium, or nickel. A second catalytic filter layeris disposed over the second metal oxide semiconductor portion, with thesecond catalytic filter layer being substantially free of the Noxcatmaterial. The first and second metal oxide semiconductor portions aredisposed in spaced relation to each other such that there is a gapbetween the first and second metal oxide semiconductor portions. Thefirst and second catalytic filter layers are disposed in spaced relationto each other. The first and second oxygen sensing circuits areconfigured to provide similar resistances when subjected to anenvironment free of nitrous oxide, but to provide substantiallydifferent resistances when subjected to an environment containingnitrous oxide(s).

In a second aspect, a nitrous oxide sensor includes a substrate; anelectrically conductive common electrode mounted on a first side of thesubstrate; an electrically conductive first sensing electrode mounted onthe first side of the substrate in spaced relation to the commonelectrode; and an electrically conductive second sensing electrodemounted on the first side of the substrate in spaced relation to thecommon electrode. The common electrode is disposed between the first andsecond sensing electrodes. The first sensing electrode and the commonelectrode are configured to form a first comb structure, and the secondsensing electrode and the common electrode are configured to form asecond comb structure. A first metal oxide semiconductor layer isdisposed over the first comb structure and forms a first semiconductorbridge between the first sensing electrode and the common electrode. Asecond metal oxide semiconductor layer is disposed over the second combstructure and forms a second semiconductor bridge interconnecting thesecond sensing electrode and the common electrode. The first and secondmetal oxide semiconductor layers are spaced from each other. A firstcatalytic filter layer is disposed over the first metal semiconductoroxide layer, and a second catalytic filter layer is disposed over thesecond metal oxide semiconductor layer. The first catalytic filter layercomprises a Noxcat material and the second catalytic filter layer issubstantially free of Noxcat material.

In a third aspect, a nitrous oxide sensor assembly includes the nitrousoxide sensor of the first aspect and/or the second aspect. In addition,the nitrous oxide sensor assembly includes a first voltage divideroperatively connected to the first oxygen sensing circuit, and a secondvoltage divider operatively connected to the second oxygen sensingcircuit, and processing circuitry. The processing circuitry is operativeto: a) determine a first voltage drop associated with the first oxygensensing circuit, the first voltage drop proportional to a resistance ofthe first oxygen sensing circuit; b) determine a second voltage dropassociated with the second oxygen sensing circuit, the second voltagedrop proportional to a resistance of the second oxygen sensing circuit;c) determine a temperature of the sensor; and d) determine if a purge ofa NOx trap associated with a combustion process is needed by determiningif a difference between the first voltage drop and the second voltagedrop is greater than a first threshold. The first threshold mayadvantageously be based on the determined temperature of the nitrousoxide sensor. If the nitrous oxide sensor includes a heater portion,such as disposed on a second side of the substrate, the processingcircuitry may be operative to determine the temperature of the nitrousoxide sensor by determining a voltage drop across a resistor disposed inseries with the heater portion. In some sub-aspects, the processingcircuitry is further operative to trigger a purge of the NOx trap inresponse to determining that the difference between the first voltagedrop and the second voltage drop is greater than the first threshold.

In a fourth aspect, any NOx sensor disclosed herein may be used todetect nitrous oxide in exhaust from a combustion process. Such a methodmay include exposing the NOx sensor to the exhaust from the combustionprocess. The NOx sensor may have first and second oxygen sensingcircuits that share a common electrode; wherein the first oxygen sensingcircuit comprises a first metal oxide semiconductor portion in contactwith the common electrode; wherein the second oxygen sensing circuitcomprises a second metal oxide semiconductor portion in contact with thecommon electrode. The method includes passing some of the exhaustthrough a first catalytic filter layer to reach the first metal oxidesemiconductor portion. The method also includes simultaneously passingsome of the exhaust through a second catalytic filter layer to reach thesecond metal oxide semiconductor portion. The method includes at leastpartially converting nitrous oxide in the exhaust to oxygen whilepassing through the first catalytic filter layer prior to reaching thefirst metal oxide semiconductor portion, but in parallel nitrous oxidein the exhaust is not converted to oxygen while passing through thesecond catalytic filter layer to reach the second metal oxidesemiconductor portion. The method also includes determining atemperature of the NOx sensor. The method also includes determining anitrous oxide level in the exhaust based on a resistance differentialbetween the first oxygen sensing circuit and the second oxygen sensingcircuit, and the determined temperature. The first and second oxygensensing circuits are configured to provide similar resistances whensubjected to an environment free of nitrous oxide, but to providesubstantially different resistances when subjected to an environmentcontaining nitrous oxide(s). The process may continue by comparing theresistance differential to a threshold, and triggering a purging of theNOx trap (e.g., providing urea/ammonia thereto) when the determined NOxlevel exceeds the threshold.

The various aspects discussed above may be used alone or in anycombination. The various apparatus disclosed herein may operateaccording to any combination of various methods disclosed herein, andvice versa. Further, the present invention is not limited to the abovefeatures and advantages. Indeed, those skilled in the art will recognizeadditional features and advantages upon reading the following detaileddescription, and upon viewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of an engine, having an NOxsensor of the present invention and/or where the method of the one ormore embodiments of the present invention may be implemented.

FIG. 2 shows a schematic representation of the electrode configurationsof one embodiment of an NOx sensor.

FIG. 3 shows the apparatus of FIG. 2 with the addition of the MOS layerand the differing porous protective (catalytic filter) layers.

FIG. 4 shows a side view of the apparatus of FIG. 3.

FIG. 5 shows a simplified schematic of the NOx sensor of FIG. 3connected to a controller via a connecting circuit.

FIG. 6 shows one multiple-cylinder configuration with a common NOxsensor for all cylinders.

FIG. 7 shows an exemplary process flow for one or more aspects.

DETAILED DESCRIPTION

In one or more aspects, the present application is directed to a methodof sensing NOx levels using a resistive based NOx sensor exposed toexhaust gases of a combustion process, and related devices and systems.In exemplary embodiments, the NOx sensor may have two sensing circuitsthat share a common electrode. The sensing circuits contain the samemetal oxide semiconductor material, but are differentiated by havingdifferent porous catalytic filter coatings thereon: one sensing circuithas a porous catalytic filter coating that contains a nitrous oxidecatalyzing or “Noxcat” material, while the porous catalytic filtercoating of the other sensing circuit is substantially free of the Noxcatmaterial. The term Noxcat material, as used herein, means any one of thefollowing materials: rhodium, ruthenium, cobalt, palladium, or nickel,which are all class VIII elements in the periodic table of elements, andwhich act as catalysts to convert NOx to N₂ and O₂ at exhaust gastemperatures. The two sensing circuits are simultaneously exposed to theexhaust gases at a common macro location. The NOx level is determinedbased on a difference in resistance between the two sensing circuits.The sensed NOx level may be used to control an operation of the exhaustcontrol system, such as triggering a purging of the NOx trap (e.g.,providing urea/ammonia thereto) when the NOx level exceeds a threshold.

In one or more aspects, the sensor element is composed of a dielectricsubstrate onto which a platinum (or other suitable material) resistor isapplied. This resistor can either be used as a means of measuring thetemperature of the sensor element in a passive state due to the stronglinear relationship between the resistance and the temperature, or(preferably) it can be used to control the temperature of the sensorelement to a narrow value by adjusting the applied voltage; in this caseit would function as a heater.

In one or more aspects, also applied to the ceramic substrate,advantageously on the opposite side, is a refractory (ex: platinum,palladium, or other suitable material) conductor consisting of at leastthree discrete electrodes. One of these electrodes is a commonelectrode, which forms two intermeshing combs with the other electrodes.One or more metal oxide semiconducting (MOS) materials are applied overthese combs to form respective semiconductor bridges. The MOS materialsmay be an n-type MOS (ex: TiO₂) and/or a p-type MOS (ex: Cr₂O₃). Bothmaterials require thermal energy (heat) to convert them from essentiallydielectric materials to semiconducting materials with the followingdistinction: a) the resistivity of n-type semiconductors increases withincreased oxygen exposure, and b) the resistivity of p-typesemiconductors decreases with increased oxygen exposure. Porous coatingsoverlay these MOS materials. The porous coatings contain variouscatalytic materials to enhance responsiveness and/or protect thesensitive MOS material from contaminants and harmful deposits.

In one or more aspects, the MOS material and geometry used on each combis advantageously the same with respect to material and geometry. Theintention is that the combs would be held at essentially the sametemperature due to their proximity to each other, and they wouldotherwise respond to the exhaust gasses to which they are exposed to inthe same way with respect to their resistance if not for a difference intheir respective porous catalytic coatings. The porous coating over oneof MOS materials contains a Noxcat material (e.g., rhodium), whereas theporous coating over the other MOS material does not contain the Noxcatmaterial. The purpose of the Noxcat material is to convert NOx to N₂ andO₂. As a result, the oxygen to which the MOS materials is exposed istherefore different, resulting in a different resistance, with theamount of the resistance difference depending upon the temperature,geometry of the sensing materials, electrodes, etc. Under theseconditions, the resistance measurement difference is indicative of thepresence of NOx, since the extra oxygen came from the conversion of NOxto N₂ and O₂. With an n-type MOS material the resistance of the Noxcatmaterial catalyzed portion will be higher than the non-Noxcat materialbearing portion, while for p-type semiconductors, the Noxcat materialbearing portion will have a lower resistance than the non-Noxcatmaterial bearing portion.

In one or more aspects, measurement of the resistance difference can becarried out in various ways including the following: Each leg of thesensor is placed in series with two resistors of known value (a voltagedivider) and a fixed DC voltage (example 5 VDC) is applied across thecircuit as shown in FIG. 5. The voltage drop across one of the tworesistors in each leg is indicative of the resistance of the sensorelement portion, which, in turn, is indicative of the state of theexhaust gas for that portion of the sensor, and the difference in theresistance is indicative of the presence of NOx in the exhaust gas.

By heating the NOx sensor and exposing it to the exhaust stream, the NOxsensor's resistance ranges will change according to the elementtemperature as well as the oxygen and NOx content of the gas to which itis exposed. The element temperature can be controlled by adjusting thevoltage to the heater to hit a target heater resistance (calculated fromapplied voltage and measured current).

In one or more aspects, the more complete system is comprised of asensor containing the sensor element described above (the threeelectrodes, the MOS material bridges, the catalytic layers, and theoptional heater), which is packaged in a housing that is durable enoughfor the exhaust system environment, circuitry housed in the electroniccontrol unit (or intermediate box) connected to the NOx sensor's heaterand oxygen-sensitive MOS portions of the element, and a logic system toconvert the range and differences in resistance along with temperatureinformation to a NOx value.

The NOx sensor and accompanying circuitry described herein hasapplicability for single and multiple combustion chamber (piston,rotary, etc. . . . ) spark ignition (SI) as well as compression ignition(diesel and HCCI) engines and burner systems (boilers, heaters, etc. . .. ) where detection of NOx is important to the emissions managementcontrol system. However, for simplicity, the discussion herein maygenerally be in the context of an NOx sensor for a small displacementgasoline powered internal combustion engine, but it should be understoodthat the NOx sensor(s) disclosed herein may be used in other internalcombustion engine applications, such as hydrogen powered engines, otherhydrocarbon powered engines, diesel engines, Homogeneous ChargeCompression Ignition (HCCI) engines, and Reactivity ControlledCompression Ignition (RCCI) engines. Further, the disclosed method(s)may be used with other combustion processes, such as, for example, thosefound in furnaces. boilers, and water heaters.

FIG. 1 shows a schematic of an exemplary internal combustion engine 10,which may be of any type (e.g., piston, rotary, nutating disk, etc.).The engine 10 includes at least one combustion chamber 12 withassociated piston, valves, etc. (not shown), an intake plenum 18, anexhaust plenum 19, and an engine management system 30. The intake plenum18 supplies air to the combustion chamber 12. A mass airflow (MAF)sensor or manifold air pressure (MAP) sensor 22, advantageously withassociated temperature sensor, is disposed in the intake plenum 18 sothat the incoming air conditions may be monitored as the airflow changesdue to the opening or closing of the throttle body 15, which is used tocontrol the load/speed of the engine, typically based on operatordemand. A controllable fuel metering system, such as a fuel injector 16,supplies fuel to the combustion chamber under control of the enginemanagement system 30. For diesel engines, the fuel injector 16 typicallysprays fuel directly into the combustion chamber 12, and ignition takesplace as a result of reaching a particular compression level. For sparkignition engines, the fuel injector 16 may spray fuel into the intakeplenum 18 or directly into the combustion chamber 12, and a sparkignition device 14, e.g., spark plug, operates under the control of theengine management system 30 to ignite the air and fuel mixture in thecombustion chamber 12 at the desired time in the cycle for propercombustion. A NOx sensor 50 is disposed in the exhaust plenum 19,downstream of a NOx trap 21 to sense the amount of NOx in the exhaustgases. Note that the NOx trap 21 may be placed in any appropriatelocation in the exhaust plenum. The engine management system 30 includesone or more processing circuits 32 (or, “processing circuitry” 32,collectively “controller”) that control the fuel supply amount andtiming, ignition timing, and other engine parameters based on the inputfrom various sensors and the programming of the processing circuits 32.For example, the engine management system 30 uses the NOx sensor 50, asdescribed below, to help control the urea injector 17 so that the engine10 operates with desired exhaust conditions. As can be appreciated, thecontroller 32 may be a centralized microprocessor, or may be adistributed architecture, where different portions of the controller 32,which may be physically distributed, handle different engine functions,such as the computations and signaling associated with the NOx sensor50, with suitable hardware, firmware, and/or software programming tocarry out the functions described herein. Other than the particulars ofthe NOx sensor 50 and the operation of the processing circuit(s) 32related to the NOx sensor 50, described in greater detail below, theconfiguration and operations of the engine 10 are well known to those ofskill in the art, and are not discussed further herein in the interestsof brevity.

Referring now to FIGS. 2-4, the NOx sensor 50 may be a resistive-basedNOx sensor, with an exhaust/NOx sensing portion 60 mounted to adielectric substrate 52. The dielectric substrate 52 may be a firedceramic dielectric substrate of any suitable type known in thesemiconductor art. The NOx sensor 50 may or may not include aresistance-based heater portion 54 (see FIG. 4), also mounted to thedielectric substrate 52, but in spaced relation to the sensing portion60.

In some embodiments, the sensing portion 60 includes a common electrode62, a first sensing electrode 70, and a second sensing electrode 80, alldisposed on a first or top side 52 a of the dielectric substrate 52. Thecommon electrode 62 includes a central area 61 and two sets of fingers64, each set including a plurality of fingers that form respective combs66,68. The combs 66,68 may advantageously extend in opposite lateraldirections from the central area 61, although this is not required. Forthe configuration shown in FIGS. 2-4, the first sensing electrode 70 isdisposed on one lateral side of the common electrode 62 in spacedrelation thereto. The first sensing electrode 70 includes a set of aplurality of fingers 72 that form a comb 74, and a terminal area 76. Thecomb 74 of the first sensing electrode 70 is advantageously intermeshedwith comb 66 of the common electrode 62 to form a comb structure, withthe respective fingers 64,72 being spaced from each other by a gap sothat there is not direct contact between the two electrodes 62,70.Likewise, the second sensing electrode 80 is disposed on the oppositelateral side of the common electrode 62 from the first sensing electrode70, and in spaced relation to the common electrode 62. The secondsensing electrode 80 includes a plurality of fingers 82 that form a comb84. The comb 84 of the second sensing electrode 80 is advantageouslyintermeshed with comb 68 of the common electrode 62 to form another combstructure, with the respective fingers 64,82 being spaced from eachother by a gap so that there is not direct contact between the twoelectrodes 62,80. Further, there is no direct contact between the firstand second sensing electrodes 70,80. Note that the electrodes 70,80 areelectrically conductive, and may be formed of any suitable material,such as platinum or palladium or other suitable refractory conductor.Note that the length and spacing of the fingers 64,72,82 of combs66,68,74,84, and their particular materials, may be adjusted as desiredfor the particular operating conditions for the NOx sensor 50.

An n-type (e.g., TiO₂) or p-type (e.g., Cr₂O₃) metal oxide semiconductor(MOS) layer 56 is placed over combs 66,74 so that the layer 56 forms asemiconductor bridge between the first sensing electrode 70 and thecommon electrode 62. Likewise, an n-type or p-type metal oxidesemiconductor layer 58 is placed over combs 68,84 so that the layer 58forms a semiconductor bridge between the second sensing electrode 80 andthe common electrode 62. Note that MOS layer 56 and MOS layer 58 shouldhave identical material composition and optionally geometries (otherthan being potential mirror images of each other), and therefore be ofthe same type (i.e., p-type or n-type), and isolated from each other (nocontact, e.g., with a gap between them). The layers 56,58 areadvantageously sintered to their respective electrodes 62,70,80 toensure good physical and electrical contact. Relevant to the discussionbelow, it should be understood that an n-type semiconductor has aresistance that is significantly lower and has a positive relationshipwith oxygen content when exposed to exhaust gases from a combustionprocess operating in the rich region, while the resistance is orders ofmagnitude higher and may be relatively uncorrelated to the oxygencontent in the lean region. Conversely, a p-type semiconductor has aresistance that is significantly lower and has a negative relationshipwith oxygen content when exposed to exhaust gases from a combustionprocess operating in the lean region, while the resistance is orders ofmagnitude higher and may be relatively uncorrelated to the oxygencontent in the rich region.

Respective porous dielectric protective coating layers 59 a,59 b areplaced over the semiconductor layers 56,58, and neighboring portions ofthe electrodes 62,70,80. These porous dielectric protecting coatinglayers 59 a,59 b may sometimes be referred to herein as catalytic filterlayers 59 a,59 b. These catalytic filter layers 59 a,59 b are distinctfrom each other and are therefore separated by a corresponding gap 57.Note that the “gaps” described herein, including gap 57 may be emptyvoids, the gaps can alternatively be entirely/partially filled withsuitable material, such as a dielectric frit or glaze material, as isdesired. Both catalytic filter layers 59 a,59 b may include catalyticprecious metal(s), such as platinum, and/or palladium, as well as oxygenstorage components such as cerium oxide or other suitable material asmay be necessary to achieve the desired functional characteristics ofthe sensing portion 60. These catalytic materials may be an initial partof the composition of the catalytic filter layers 59 a,59 b, or added asto impregnate the catalytic filter layers 59 a,59 b in a subsequentoperation. In addition, one of the catalytic filter layers, for examplecoating layer 59 a, contains Noxcat material (at a defined level, suchas 0.25% to 2.5%), while the other catalytic filter layers (in thisexample 59 b) is substantially free of the Noxcat material.Substantially free, in this context, means that the concentration of theNoxcat material is approximately zero, and at least low enough not toconvert any substantial amounts of NOx to N₂ and O₂, so as to notsubstantially change the resistance of the associated MOS layer due tothe presence of NOx in the exhaust gas (as compared to an identicalexhaust gas without the NOx).

Referring to FIGS. 4-5, a first oxygen sensing circuit 42 is formed bythe common electrode 62, the MOS layer 56, the first sensing electrode70. The first oxygen sensing circuit 42 includes the MOS layer 56 havingthe catalytic filter layer 59 a of a first type, and is thereforeassociated with the catalytic filter layer 59 a of the first type. Asecond oxygen sensing circuit 44 is formed by the common electrode 62,the MOS layer 58, the second sensing electrode 80. The second oxygensensing circuit 44 includes the MOS layer 58 having the catalytic filterlayer 59 b of a second type, and is therefore associated with thecatalytic filter layer 59 b of the second type. In an illustrativeexample, the catalytic filter layer of the first type comprises Noxcatmaterial, while the catalytic filter layer of the second type issubstantially Noxcat material free. In another illustrative example, thecatalytic filter layer of the first type is substantially Noxcatmaterial free, and the catalytic filter layer of the second typecomprises Noxcat material.

When Noxcat material is present in the catalytic filter layer (e.g. 59a), and that catalytic filter layer is exposed to the exhaust streamhaving NOx present, the Noxcat material interacts with NOx to produce N₂and O₂. Thus, in an example where the MOS layer 56 is covered by theporous catalytic filter layer with Noxcat material (59 a), while MOSlayer 58 is covered by a porous catalytic filter layer that issubstantially Noxcat material free (59 b), MOS layer 56 responds, whenNOx is present in the exhaust plenum 19 at the location of the NOxsensor 50, in a way so as to indicate a higher oxygen level than the MOSlayer 58 covered by the porous layer without Noxcat material (59 b). Forexample, when MOS layers 56,58 are both n-type semiconductors, then MOSlayer 56 has a significantly higher resistance than MOS layer 58, when,in the absence of NOx, the two layers 56/58 would have substantiallysimilar resistances. Therefore, despite being exposed to a commonexhaust having a certain (and shared) actual oxygen level, the twosensor circuits 42,44 provide different resistances, with the differencein resistance proportional to the amount of NOx present in the exhaustplenum 19 after the NOx trap 21.

In some, but not all, embodiments, an optional heater portion 54 isadvantageously disposed on the bottom side 52 b of the dielectricsubstrate 52, generally opposite from the exhaust sensing portion 60.See FIG. 4. In other embodiments (not shown), the heater portion 54 maybe disposed on the same side 52 a of the dielectric substrate 52 as theexhaust sensing portion 60, provided the two portions 54, 60 areelectrically isolated (not electrically connected other than throughground). This can be achieved by applying a dielectric layer (not shown)over the heater portion 54 via screen printing, vapor deposition, orother techniques known in the art. In still other embodiments, theheater portion 54 is omitted. The heater portion 54 may take anysuitable form and may be formed of any suitable material such astungsten, platinum, palladium, other precious metals, ceramic (such assilicon nitride Si₃N₄), or other materials known in the art. ReferencingFIGS. 4-5, the heater portion 54 may be advantageously connected toground and to a voltage source, such as a twelve volt DC voltage sourceVS₁. The heater portion 54 may also have a suitable protective and/orelectrically insulating layer 59, as desired.

The NOx sensor 50 may have suitable connections for power and othersignals. For example, in some embodiments, the NOx sensor 50 has fourcontacts or leads 55, 69, 79, 89 for making suitable connections. Lead55 is electrically connected to the heater portion 54, and functions asa power (+) lead for the heater portion 54. Lead 69 is electricallyconnected to the common electrode 62, and acts as a ground (−) lead forthe sensing portion 60. Lead 79 is electrically connected to the firstsensing electrode 70, and acts as the output from first sensing circuit42. Lead 89 is electrically connected to the second sensing electrode80, and acts as the output from second sensing circuit 44. Lead 69 mayalso function as a ground lead for heater portion 54, or there may be anadditional lead (not shown) for a separate ground lead for heaterportion 54.

Together, the NOx sensor 50 and the controller 32 form a nitrous oxidesensing assembly or system 5. In some embodiments, the NOx sensorassembly 5 is such that the NOx sensor 50 is connected directly to thecontroller 32 so that the sensed oxygen level data from the NOx sensor50 is supplied to the controller 32. In other embodiments, the NOxsensor assembly 5 includes a connecting circuit 90 that interconnectsthe NOx sensor 5 to the computational portions of controller 32. SeeFIG. 5. Note that the connecting circuit 90, if present, may be asub-portion of the controller 32, be a separate component or componentsbetween the NOx sensor 50 and the controller 32, or dispersed in anysuitable manner. In one or more illustrative embodiments, the connectingcircuit 90 includes voltage source VS₁, which may be, as illustrated, atwelve volt DC voltage source. Voltage source VS₁ connects to one sideof heater portion 54 via shunt resistor R_(HS) and lead 55. Suitableleads disposed on either side of shunt resistor R_(HS) allow the voltagedrop V_(HS) across shunt resistor R_(HS) to be measured. Note that theline L_(VHS) for V_(HS), as with other voltage drop lines herein, isillustrated and discussed as being a single line merely for the sake ofsimplicity, but are typically two lines each. The opposing end of heaterportion 54 connects to ground via lead 69.

The connecting circuit 90 also includes a constant voltage source CV,which will be assumed to be powered using a nominal voltage of fivevolts or any other suitable stable power source that is available. Theconstant voltage source CV connects to the first sensing circuit 42 viaresistors R₁ and R₂ and lead 79. As can be appreciated, resistors R₁ andR₂ form a first voltage divider, with the voltage drop V₂ acrossresistor R₂ measured and provided by line L_(V2). The sensing circuit 92is completed to ground via the first sensing circuit 42 (e.g., firstsensing electrode 70, MOS layer 56, and common electrode 62). Theconstant voltage source CV connects to the second sensing circuit 44 viaresistors R₃ and R₄ and lead 89. As can be appreciated, resistors R₃ andR₄ form a second voltage divider, with the voltage drop V₄ acrossresistor R₄ measured and provided by line L_(V4). The sensing circuit 94is completed to ground via second sensing circuit 44 (e.g., sensingelectrode 80, semiconductor layer 58, and common electrode 62). Notethat in alternate embodiments, resistor R₁ and/or resistor R₃ may beomitted from their respective circuits, or additional resistors may beadded to their respective circuits for scaling or calibration purposes.

The connecting circuit 90 provides voltage drop V₂ to the controller 32via line L_(V2), voltage drop V_(HS) to controller 32 via line L_(VHS),the actual voltage V_(S) of voltage source VS₁ to controller 32 via lineL_(VS), and voltage drop V₄ to the controller 32 via line L_(V4).

The controller 32 may determine the NOx level based on the voltagelevels supplied to it by the sensing circuits 42,44. In particular, thecontroller 32 may determine the voltage difference between V₂ and V₄,and then determine the NOx level based thereon, advantageously takinginto account the temperature of the NOx sensor 50.

The temperature of the NOx sensor 50 may be determined by controller 32based on the resistance of the heater portion 54. For example, thecurrent I_(H) in the heater portion 54 may be calculated as the voltagedrop V_(HS) across the shunt resistor R_(HS), divided by the resistanceof the shunt resistor R_(HS), or I_(H)=V_(HS)/R_(HS). Then, theresistance R_(H) of the heater portion 54 may be calculated based on thevoltage drop across the heater portion 54 divided by the current I_(H)through the heater portion 54. Thus, R_(H) may be calculated asR_(H)=(V_(S)−V_(HS))/I_(H). Then, using R_(H), temperature T may becalculated using a suitable formula, for example T=(M×R_(H))+B, wherethe slope M and the constant B are dependent on the heater portion 54design. As can be appreciated, M and B can be determined in acalibration process, and the relevant values stored in memory of theengine management system 30 for use by the controller 32.

The controller 32 receives the inputs derived from the NOx sensor 50 andother sensors, and advantageously controls the operation of the ureainjector 17, and optionally other functions such a fuel metering,ignition timing, and other engine functions.

The discussion above has generally been in the context of thetemperature of the NOx sensor 50 being derived from the resistance ofthe heater portion 54 that is part of the NOx sensor 50. Thus, theheater portion 54 fills two roles: heating the NOx sensor 50 and sensingtemperature thereof. However, in some embodiments, a temperature sensordistinct from the heater portion 54 may alternatively employed. Thus,the NOx sensor 50 may include a thermocouple or other suitabletemperature sensing device 51, in addition to the exhaust sensingportion 60 and the optional heater portion 54. Such a temperature sensor51 is shown in FIG. 5 in dashed lines to indicate its optional presence,with its connection line to controller 32 not shown for simplicity. And,the temperature of the NOx sensor 50 may be determined by controller 32based on data from the temperature sensor 51 in a conventional fashion,either as an alternative or in addition to determining the temperaturebased on the resistance of the heater portion 54.

The discussion above has generally been in the context of controlling anengine 10 having a single cylinder/combustion chamber. However, asimilar approach may be used with engines having multiple cylinders,such as that shown in FIG. 6 with cylinders J, K, L, and P. In FIG. 6, asingle common NOx sensor 50 is used for multiple cylinders.

The discussion above has generally been in the context of an internalcombustion engine; however, the present invention is not limited inapplication to internal combustion engines. Indeed, the NOx sensingmethod described above can be used to control NOx reduction processesgenerally. Thus, for example, the method(s) described herein may be usedfor NOx measurement and NOx reduction in combustion processes in afurnace or a water heater. As with the engine-based discussion above,the NOx sensor 50 is disposed so as to sense exhaust gases in theexhaust plenum 19 from the combustion process.

From the above, it can be seen that the NOx sensor 50 may be used todetect nitrous oxide in exhaust from a combustion process. For example,as shown in FIG. 7, one method for detecting nitrous oxide in exhaustfrom a combustion process is to expose the NOx sensor 50 to the exhaustfrom the combustion process (step 110). As discussed above, the NOxsensor 50 has first and second oxygen sensing circuits 42,44 that sharea common electrode 62; wherein the first oxygen sensing circuit 42comprises a first metal oxide semiconductor portion 56 in contact withthe common electrode 62; wherein the second oxygen sensing circuit 44comprises a second metal oxide semiconductor portion 58 in contact withthe common electrode 62. Some of the exhaust passes through a firstcatalytic filter layer 59 a (step 120) to reach the first metal oxidesemiconductor portion 56. Simultaneously, some of the exhaust passesthrough a second catalytic filter layer 59 b (step 130) to reach thesecond metal oxide semiconductor portion 58. At step 120, nitrous oxidein the exhaust is converted at least partially to oxygen while passingthrough the first catalytic filter layer 59 a prior to reaching thefirst metal oxide semiconductor portion 56; but during parallel step130, nitrous oxide in the exhaust is not converted partially to oxygenwhile passing through the second catalytic filter layer 59 b to reachthe second metal oxide semiconductor portion 58. The process alsoincludes, at step 135, determining the temperature of the NOx sensor,which may be accomplished in any suitable way, such as calculated fromthe heater current I_(H) or the voltage drop across a resistor R_(HS)disposed in series with the heater portion 54, or be based on data froma separate temperature sensor 51. Note that step 135 may occur before orafter or concurrent with step 120 and 130. The process continues, atstep 140, with determining a nitrous oxide level in the exhaust based ona resistance differential between the first oxygen sensing circuit 42and the second oxygen sensing circuit 44 and the determined temperatureof the NOx sensor. As pointed out above, the first and second oxygensensing circuits 42,44 are configured to provide similar resistanceswhen subjected to an environment free of nitrous oxide, but to providesubstantially different resistances when subjected to an environmentcontaining nitrous oxide(s). The process may continue to the optionalstep of comparing the resistance differential to a threshold (step 150),and triggering a purging of the NOx trap 21 (e.g., providingurea/ammonia thereto) (step 160) when the determined NOx level exceedsthe threshold. The threshold may advantageously vary based on thedetermined temperature of the sensor, so as to compensate for thetemperature dependent response of the MOS materials of layers 56,58 tothe presence of oxygen.

The discussion above has generally been in the context of catalyticfiler layer 59 a having a particular Noxcat material, while the othercatalytic filter layer 59 b is substantially free of that particularNoxcat material. Thus, catalytic filer layer 59 a may comprise rhodium,while catalytic filter layer 59 b is substantially free of rhodium. Thisis an example using a single Noxcat material, and is believed to be anadvantageous configuration. However, in other aspects or embodiments,catalytic filer layer 59 a may comprise a combination of multiple Noxcatmaterials (e.g., rhodium and nickel), while catalytic filter layer 59 bis substantially free of those Noxcat materials. Claim language of“wherein the first catalytic filter layer comprises a Noxcat material .. . wherein the second catalytic filter layer is substantially free ofthe Noxcat material,” and the like, is intended to cover both situations(single Noxcat material and combination of two Noxcat materials), andsituations where more Noxcat materials are involved.

The methods and engine control systems discussed above provide theopportunity for enhanced combustion and/or engine control so thatgreater fuel economy and/or reduced emissions may be achieved.

For more information about forming the electrodes 62,70,80 and the MOSlayers 56,58, and the like, and/or temperature compensation of thesensed oxygen levels, see U.S. Patent Application Publication20140130779 and/or U.S. Pat. Nos. 8,586,394 and 8,959,987, thedisclosures of which are incorporated herein by reference in theirentirety.

The present invention may, of course, be carried out in other specificways than those herein set forth without departing from the scope of theinvention. The present embodiments are, therefore, to be considered asillustrative and not restrictive.

What is claimed is:
 1. A nitrous oxide sensor assembly, comprising: anitrous oxide sensor comprising: a substrate; a first oxygen sensingcircuit affixed on a first side of the substrate; a second oxygensensing circuit affixed on the first side of the substrate; wherein thefirst oxygen sensing circuit comprises: a first sensing electrode; acommon electrode; a first metal oxide semiconductor portion electricallybridging a first physical gap between the first sensing electrode andthe common electrode; wherein the second oxygen sensing circuitcomprises: a second sensing electrode; the common electrode; a secondmetal oxide semiconductor portion electrically bridging a secondphysical gap between the second sensing electrode and the commonelectrode; wherein a first catalytic filter layer is disposed over thefirst metal oxide semiconductor portion; wherein the first catalyticfilter layer comprises a Noxcat material; wherein the Noxcat material isone of rhodium, ruthenium, cobalt, palladium, or nickel; wherein asecond catalytic filter layer is disposed over the second metal oxidesemiconductor portion; wherein the second catalytic filter layer issubstantially free of the Noxcat material; wherein the first and secondmetal oxide semiconductor portions are disposed in spaced relation toeach other such that there is a third physical gap between the first andsecond metal oxide semiconductor portions; wherein the first and secondcatalytic filter layers are disposed in spaced relation to each other;wherein the first and second oxygen sensing circuits are configured toprovide similar resistances when subjected to an environment free ofnitrous oxide, but to provide substantially different resistances whensubjected to an environment containing nitrous oxide(s); a first voltagedivider operatively connected to the first oxygen sensing circuit; asecond voltage divider operatively connected to the second oxygensensing circuit; processing circuitry operative to: determine a firstvoltage drop associated with the first oxygen sensing circuit, the firstvoltage drop proportional to a resistance of the first oxygen sensingcircuit; determine a second voltage drop associated with the secondoxygen sensing circuit, the second voltage drop proportional to aresistance of the second oxygen sensing circuit; determine a temperatureof the nitrous oxide sensor; determine if a purge of a NOx trapassociated with a combustion process is needed by determining if adifference between the first voltage drop and the second voltage drop isgreater than a first threshold.
 2. The nitrous oxide sensor assembly ofclaim 1, wherein the Noxcat material is rhodium.
 3. The nitrous oxidesensor assembly of claim 1, further comprising a heater portion disposedon a second side of the substrate, the second side opposite the firstside.
 4. The nitrous oxide sensor assembly of claim 1, wherein the firstand second metal oxide semiconductor portions are both n-typesemiconductors, or are both p-type semiconductors.
 5. The nitrous oxidesensor assembly of claim 1, wherein the first and second sensingelectrodes are mirror images of each other.
 6. The nitrous oxide sensorassembly of claim 1, wherein the gap is an empty void.
 7. The nitrousoxide sensor assembly of claim 1, wherein the first threshold is basedon the determined temperature of the nitrous oxide sensor.
 8. Thenitrous oxide sensor assembly of claim 7: wherein the nitrous oxidesensor further comprises a heater portion disposed on a second side ofthe substrate; wherein the processing circuitry is operative todetermine the temperature of the nitrous oxide sensor by determining avoltage drop across a resistor disposed in series with the heaterportion.
 9. The nitrous oxide sensor assembly of claim 1, wherein theprocessing circuitry is further operative to trigger a purge of the NOxtrap in response to determining that the difference between the firstvoltage drop and the second voltage drop is greater than the firstthreshold.
 10. A method of detecting nitrous oxide in exhaust from acombustion process, the method comprising: exposing a nitrous oxidesensor to the exhaust from the combustion process; wherein the nitrousoxide sensor constitutes a portion of a nitrous oxide sensor assembly,the nitrous oxide sensor comprising: a substrate; a first oxygen sensingcircuit affixed on a first side of the substrate; a second oxygensensing circuit affixed on the first side of the substrate; wherein thefirst oxygen sensing circuit comprises: a first sensing electrode; acommon electrode; a first metal oxide semiconductor portion electricallybridging a first physical gap between the first sensing electrode andthe common electrode; wherein the second oxygen sensing circuitcomprises: a second sensing electrode; the common electrode; a secondmetal oxide semiconductor portion electrically bridging a secondphysical gap between the second sensing electrode and the commonelectrode; wherein a first catalytic filter layer is disposed over thefirst metal oxide semiconductor portion; wherein the first catalyticfilter layer comprises a Noxcat material; wherein the Noxcat material isone of rhodium, ruthenium, cobalt, palladium, or nickel; wherein asecond catalytic filter layer is disposed over the second metal oxidesemiconductor portion; wherein the second catalytic filter layer issubstantially free of the Noxcat material; wherein the first and secondmetal oxide semiconductor portions are disposed in spaced relation toeach other such that there is a third physical gap between the first andsecond metal oxide semiconductor portions; wherein the first and secondcatalytic filter layers are disposed in spaced relation to each other;wherein the first and second oxygen sensing circuits are configured toprovide similar resistances when subjected to an environment free ofnitrous oxide, but to provide substantially different resistances whensubjected to an environment containing nitrous oxide(s); the nitrousoxide sensor assembly further comprising: a first voltage divideroperatively connected to the first oxygen sensing circuit; a secondvoltage divider operatively connected to the second oxygen sensingcircuit; processing circuitry operative to: determine a first voltagedrop associated with the first oxygen sensing circuit, the first voltagedrop proportional to a resistance of the first oxygen sensing circuit;determine a second voltage drop associated with the second oxygensensing circuit, the second voltage drop proportional to a resistance ofthe second oxygen sensing circuit; determine a temperature of thenitrous oxide sensor; determine if a purge of a NOx trap associated witha combustion process is needed by determining if a difference betweenthe first voltage drop and the second voltage drop is greater than afirst threshold; passing some of the exhaust through the first catalyticfilter layer to reach the first metal oxide semiconductor portion, andsimultaneously passing some of the exhaust through the second catalyticfilter layer to reach the second metal oxide semiconductor portion;wherein nitrous oxide in the exhaust is at least partially converted tooxygen while passing through the first catalytic filter layer prior toreaching the first metal oxide semiconductor portion, but nitrous oxidein the exhaust is not converted to oxygen while passing through thesecond catalytic filter layer to reach the second metal oxidesemiconductor portion; determining, by the processing circuitry, atemperature of the nitrous oxide sensor; and determining, by theprocessing circuitry, a nitrous oxide level in the exhaust based on aresistance differential between the first oxygen sensing circuit and thesecond oxygen sensing circuit and the determined temperature.
 11. Themethod of claim 10, wherein the exposing the nitrous oxide sensor to theexhaust comprises exposing the nitrous oxide sensor to the exhaust at apoint downstream from a NOx trap.
 12. The method of claim 10, furthercomprising, comparing the resistance differential to a threshold, thethreshold varying based on the determined temperature of the nitrousoxide sensor.
 13. The method of claim 12, further comprising, inresponse to the resistance differential exceeding the threshold,triggering a purging of a NOx trap disposed along an exhaust associatedwith the combustion process.
 14. The method of claim 10: wherein thenitrous oxide sensor further comprises a heater portion disposed on asecond side of the substrate, the second side opposite the first side;wherein the determining the temperature of the sensor comprisesdetermining the temperature based on a resistance of the heater portion.15. The method of claim 10: wherein the nitrous oxide sensor furthercomprises a heater portion disposed on a second side of the substrate;wherein the determining the temperature comprises determining thetemperature based on a temperature sensor distinct from the heaterportion.
 16. The method of claim 10, wherein the first and second metaloxide semiconductor portions are both n-type semiconductors.
 17. Themethod of claim 10, wherein the first and second metal oxidesemiconductor portions are both p-type semiconductors.
 18. The method ofclaim 10, wherein the first and second sensing electrodes are mirrorimages of each other.